1. Field of the Invention
The invention relates to a method of depositing a composite coating comprising a metallic matrix containing particles, for the purpose of repairing a metal blade, particularly but not exclusively a blade of a gas turbine nozzle.
The invention relates in particular to a method of depositing a coating of the M1CrAlM2 type, where M1 is selected from Ni, Co, or Fe, or a mixture thereof, and M2 is selected from Y, Si, Ti, Hf, Ta, Nb, Mn, Pt, and rare earths.
2. Description of the Related Art
The continuing improvement in the efficiency of modern gas turbines makes it necessary to use inlet temperatures to the turbine that are ever higher. This trend has led to ever more refractory materials being developed to make the parts of the high pressure turbine such as moving blades and nozzles.
For this purpose, monocrystalline superalloys have been developed with very high volume fractions of the gamma prime phase that presents hardening properties.
Nevertheless, the development of superalloys no longer suffices for keeping up with the increasing requirements in terms of lifetime for parts that withstand high temperatures. That is why, more recently, thermally insulating coatings have come into service for lowering the temperature of the metal of parts that are cooled by internal convection. These thermally-insulating coatings or “thermal barriers” are made of a layer of ceramic based on zirconia stabilized by yttrium oxide and deposited on a metallic bonding layer to provide adhesion for the ceramic coating while protecting the metal of the part from being oxidized.
The bonding layer, referred to as an undercoat, may be of various types. Mention can be made of layers of the MCrAlY type (where M stands for nickel or cobalt). Mention can be made in particular of layers of the aluminide (NiAl) type having an intermetallic structure, compounds which are defined as having 50% atomic of nickel and aluminum. Such aluminides may be modified by a precious metal such as platinum. Aluminide coatings are made up of an outer layer formed together with a layer that diffuses into the substrate. All those undercoat systems have as their common denominator the property of being alumina-forming, i.e. by oxidizing they form a protective alumina film that adheres well and that isolates the metal of the part from the oxidizing environment.
In spite of all the protections added to parts, such as undercoats and thermal barriers, they nevertheless oxidize and they run the risk of cracking. In order to enable such parts to continue to be used, it is therefore necessary to repair the various defects they might present after a certain length of service.
In order to repair a part such as a nozzle coated with a thermal barrier, it is known to be necessary to remove the ceramic coating and then the metallic undercoat. It is then necessary to deoxidize the part by thermal and chemical treatment under a halogen atmosphere. The part can then be repaired by a welding and/or brazing technique. Once the part has been built up, the metallic undercoat is restored and then the ceramic layer.
The thermal barrier is conventionally removed by sand blasting. Sand blasting is an operation that is aggressive both to the ceramic layer and to the metallic undercoat. The undercoat is subsequently removed by being dissolved chemically in a bath of acid. That operation is difficult since it leads to the diffused layer of the aluminide coating being dissolved and thus it leads in practice to reducing the wall thicknesses of the part. Such a reduction in the wall thickness of the parts leads to an increase in flow section, in particular for nozzles.
In a turbomachine nozzle, a sector is a part comprising one or more blades mounted on interconnected platforms. Sectors are united to form a ring that essentially constitutes the nozzle. Strictly-speaking, the flow section of a sector is the area, measured perpendicularly to the flow direction, of the passage along which the stream passes through the nozzle sector, between two adjacent blades. By extension, the flow section is used to designate more simply the width of the passage for the stream through the nozzle sector. This flow section is conventionally measured at that location between the leading edge and the trailing edge at which the value of the flow section is the smallest, which corresponds to the location of the narrowest passage for the stream.
It is known that when the flow section is increased, that tends to diminish the performance of an engine by diminishing the exhaust gas temperature (EGT) margin.
It is therefore necessary to be in a position to add material at the location where the part determines the performance of the engine, while conserving good mechanical properties and the ability to withstand oxidation and corrosion.
The traditional technology comprises building up the part by brazing on a frit based on superalloy and a brazing material. That technology is not particularly suitable since it presents various drawbacks.
By definition, frits and brazing powders are made of meltable elements that form compounds having a melting point close to the operating temperature of the parts. It is therefore not recommended to use materials of this kind over large areas that are exposed to extreme temperatures. As a result, the mechanical characteristics of brazed zones are well below those of bare substrates.
Furthermore, making a deposit by brazing always leads to an edge that forms a step, i.e. an extra thickness of material all along the built-up zone. The presence of this step can disturb the flow of the stream of air (in the air flow section), so subsequent machining is necessary to restore the proper aerodynamic profile.
Furthermore, it can happen that the trailing edge of the nozzle is not thick enough for it to be brazed: brazing is accompanied by elements being diffused over thicknesses that may be as great as 300 micrometers (μm) and that therefore degrade the integrity of the substrate over said thickness.